The letter discusses the role of endomyocardial biopsy in the management of cardiovascular disease, particularly in patients with suspected myocardial siderosis due to hereditary or acquired hemochromatosis. The authors highlight that cardiac involvement in hemochromatosis can often be diagnosed through history, clinical examination, and imaging techniques like echocardiography or cardiac magnetic resonance (CMR). However, cardiac siderosis may present late, and conventional markers for iron overload, such as serum ferritin and liver iron, do not always correlate with myocardial iron deposition in conditions like beta-thalassemia major. The authors propose that myocardial biopsy should only be performed if access to CMR and measurement of T2* (a technique that measures myocardial iron load) is not available or if other etiologies are being assessed. They emphasize that CMR, particularly the T2* technique, is a robust, simple, and quick method for assessing myocardial iron load and has been validated across different scanners from major manufacturers. In response, Dr. Lisa Anderson from St. George's Hospital and Dudley Pennell from the CMR Unit at Royal Brompton Hospital acknowledge the clinical merits of CMR-measured myocardial T2* in evaluating cardiac function. They also note that while there is no direct calibration of myocardial T2* against absolute myocardial iron levels in humans, CMR has been validated in animal models, suggesting that the technique may not need to be calibrated for human use.The letter discusses the role of endomyocardial biopsy in the management of cardiovascular disease, particularly in patients with suspected myocardial siderosis due to hereditary or acquired hemochromatosis. The authors highlight that cardiac involvement in hemochromatosis can often be diagnosed through history, clinical examination, and imaging techniques like echocardiography or cardiac magnetic resonance (CMR). However, cardiac siderosis may present late, and conventional markers for iron overload, such as serum ferritin and liver iron, do not always correlate with myocardial iron deposition in conditions like beta-thalassemia major. The authors propose that myocardial biopsy should only be performed if access to CMR and measurement of T2* (a technique that measures myocardial iron load) is not available or if other etiologies are being assessed. They emphasize that CMR, particularly the T2* technique, is a robust, simple, and quick method for assessing myocardial iron load and has been validated across different scanners from major manufacturers. In response, Dr. Lisa Anderson from St. George's Hospital and Dudley Pennell from the CMR Unit at Royal Brompton Hospital acknowledge the clinical merits of CMR-measured myocardial T2* in evaluating cardiac function. They also note that while there is no direct calibration of myocardial T2* against absolute myocardial iron levels in humans, CMR has been validated in animal models, suggesting that the technique may not need to be calibrated for human use.